The achievement of high efficiencies for solar cells on large area substrates is dependent upon achieving narrow line widths and high aspect ratios (ratio of height to width) for the metallization on the surface exposed to sunlight. The buried contact solar cell [S. R. Wenham and M. A. Green, U.S. Pat. Nos. 4,726,850 and 4,748,130] provides a design for the metallization that achieves both fine line widths and high aspect ratios for the metallization while simultaneously being well suited to commercial production. However, previous implementations of the buried contact solar cell metallization scheme have required the deposition or growth of a dielectric or equivalent layer across the top surface of the substrate early in the processing sequence so as to provide masking of the top surface against diffusions, chemical etching, and metal plating. By necessity, this dielectric masking layer has had to be able to withstand the chemical etching and the high temperature processing without deterioration or excessive thinning. This has greatly limited the available choice for such dielectrics, particularly when the dielectric is then required to also act as an anti-reflection coating at the end of the process.
A typical implementation previously reported in the literature involves the formation of the dielectric layer across the top surface prior to laser scribing. The laser scribing then forms grooves through the dielectric layer and into the silicon material. A chemical etch is then used to clean and etch the grooves while the masking dielectric layer across the top surface prevents the etching solution having any impact on the top surface. A high temperature process is then used during which dopants are diffused into the exposed silicon groove walls during which again the dielectric layer masks the top surface to prevent unwanted dopants from penetrating into the silicon. In this step, clearly the dielectric masking layer must not only be able to withstand the high temperatures involved, but also be able to block the diffusion of dopants through the layer without significant degradation in thickness, chemical resistance, or optical properties. Following the groove diffusion further high temperature treatments are usually required associated with the design of the rear metal contact, following which metal plating of the front surface grooves and the rear surface metal contact are effected.
In the present invention, many of the limitations and requirements for the top surface dielectric masking layer are relaxed, while the overall processing sequence is simultaneously simplified.